In recent years, a number of techniques have been proposed for extending the refrigerated shelf life of food products which might contain pathogenic bacteria, spoilage bacteria, or both. Swartzel et al., U.S. Pat. No. 4,808,425, for example, suggests that extended refrigerated shelf life on the order of four weeks or more can be imparted to foods such as, for example, liquid whole egg. According to Swartzel et al., this can be accomplished by combining so-called "ultrapasteurization" conducted on continuous flow, high temperature, short time pasteurization equipment with aseptic packaging. Although it is somewhat difficult to determine exactly what holding temperature and time constitute "ultrapasteurization", from the discussion contained in Swartzel et al., it is clear from a review of the examples provided in Table II thereof that heating was conducted on conventional high temperature, short time commercial plate-type heat exchange thermal processing systems at temperatures in excess of 146.degree. F. and up to approximately 162.degree. F. and holding times ranging from about 2.7 seconds to about 3.2 minutes.
Dunn et al., U.S. Pat. No. 4,838,154 and its parent, U.S. Pat. No. 4,695,472 take a different approach. Dunn et al. discuss methods and apparatus for extending the shelf life of fluid food products including eggs by the repeated application of high voltage, high current density, discrete electric pulses to food products. Field strengths used are, at minimum, 5,000 volts/cm and voltages as high as, for example, 37,128 volts are disclosed. Direct current densities of at least about 12 amps/cm.sup.2 are also disclosed as are pulse frequencies of between about 0.1 and 100. Preferably, the treatment involves the application of at least two and, more preferably, at least five discrete high energy pulses to the food being treated.
Dunn et al. suggest that different forms of energy, e.g. the application of different types of pulses, can result in different effects on the treated food. For example, Dunn et al. disclose the use of flat-top electric field pulses where heating is to be kept to a minimum, and exponentially decaying pulses where heating by electric field is beneficial.
Dunn et al. recognize the problems associated with electrolysis of the electrodes and the food being treated during the application of the pulsed energy high energy fields. Dunn et al. therefore propose a device including plate electrodes separated from the food being treated by a membrane and having an electrolyte placed therebetween in an attempt to mitigate this problem.
Dunn et al. teach that an extended refrigerated shelf life may be obtained in egg containing preservatives by the use of 30 pulses of electric energy having a peak voltage of 36,000 volts or more, a peak current of at least 9,600 amps and a final temperature of 136.4.degree. F. Dunn et al. suggest, based on this test, that fluid egg product containing additives which is elevated in temperature to approximately 60.degree. C. and stored at temperatures of about 4.degree. C. can provide liquid egg with an extended refrigerated shelf life of 28 days or more.
The additives used in accordance with Dunn et al. are potassium sorbate and citric acid which are termed chemical preservatives. The effects of such additives on the refrigerated shelf life of liquid egg is demonstrated in FIG. 13 of Dunn et al. where a control which was treated using pulsed electric fields but which did not contain additives (1302-control) is compared to liquid egg containing additives which had been similarly treated and stored (1306-treated). A significant shelf life extension appears to be directly attributable to the presence of these additives.